Malic enzymes catalyze the oxidative decarboxylation of malate to form pyruvate and CO2. Two types of these enzymes are known to occur in animal cells. Both cytosol and mitochondria may contain a strictly NADP-dependent form. Mitochondria only may also contain a NAD(P)-linked enzyme. The latter enzyme, which we have purified about 100-fold from adrenal cortex mitochondria, is an allosteric enzyme both electrophoretically and kinetically different from the NADP-linked forms. Mitochondria from most normal tissues contain either both NAD(P)-and the NADP-dependent forms or only the NADP-dependent malic enzyme. Mitochondria from ascites tumors, however, contain only the allosteric NAD(P)-dependent enzyme. Moreover, this enzyme has a higher specific activity in tumor mitochondria compared to normal mitochondria. All ascites tumors that we have examined are deficient in both the cytosol and mitochondrial NADP-linked malic enzymes. They differ, therefore, from normal cells in this important respect, as well. A regulatory NAD(P)-dependent malic enzyme in tumor mitochondria would augment the oxidative pathway of malignant cells. For example, this enzyme would serve as a pyruvate source for continued citric acid cycle activity during glutamine and glutamate oxidation. The allosteric stimulation of the tumor NAD(P)-dependent malic enzyme by fumarate and succinate and inhibition by ATP place the enzyme at a strategic control point in glutamine oxidation. We will investigate this pathway in vitro using isolated tumor mitochondria and intact ascites tumor cells. The outcome will be a clearer understanding of the intermediary metabolism of tumors.